Add a new wreck plugin to set CUDA_VISIBLE_DEVICES when there
are GPU resources set in R_lite. The plugin also sets
CUDA_DEVICE_ORDER=PCI_BUS_ID so that CUDA uses the same GPU ids
as are understood by hwloc and the Flux scheduler.
By default, CUDA_VISIBLE_DEVICES is set to all locally allocated
GPUs in all local tasks. The list of GPUs can be partitioned and
assigned per-task with the wreck option `-o gpubind=per-task`.
This plugin may also be disabled with the option `-o gpubind=off`
Fixes#1562Fixes#1598

When the -g, --gpus-per-task option is used and no scheduler is
loaded, add "fake" GPU resources to the generated R_lite. This
will be helpful in testing plugins and other parts of flux-core
that look for allocated GPUs in R_lite.

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